This invention relates to the operation of high-temperature fuel cells wherein oxygen supplied on the cathode side is converted into water and carbon dioxide with hydrogen and carbon monoxide at the anode with the generation of electrical power.
A method for the operation of fuel cells has been described in German Application P 38 10 113, wherein hydrogen and carbon monoxide are oxidized with oxygen for the production of electricity. In this conventional a steam reformer supplies the H.sub.2 -- and CO-rich process, fuel gas while air is the source of oxygen. The conventional method is specifically concerned with the so-called molten carbonate fuel cell (MCFC) which contains a carbonate melt as the electrolyte. On the cathode side, this electrolyte, which conducts oxygen ions, is supplied with carbon dioxide and oxygen which are converted to CO.sub.3.sup.2- ions at the cathode. The CO.sub.3.sup.2- ions, diffusing through the electrolyte, are converted at the anode with hydrogen and carbon monoxide into water and carbon dioxide. The conversion of hydrogen at the anode with oxygen to water is, however, incomplete resulting in an anode containing residual hydrogen waste gas. The residual hydrogen in the anode waste gas is utilized in the undergrate firing of a synthesis gas generator wherein, in an endothermal reaction, hydrocarbons are converted to H.sub.2 /CO synthesis gas. converte
For use in the MCFC method, the carbon monoxide of the synthesis gas can be changed externally of the fuel cell by conversion to CO.sub.2. In contrast thereto, with the use of an arrangement of a solid oxide type of fuel cell (SOFC), no such external conversion is required since the carbon monoxide is converted in the fuel cells proper, and the thus-formed carbon dioxide is exhausted with the anode waste gas. The SOFC cell is described for example in "Status of Fuel Cell Development" by E. Voss, presented at the Fuel Cell Workshop at Taormina/Sicily on June 6, 1987.
Another known type of cell concerns fuel cells with internal reforming. On the occasion of the "International Seminar, The Netherlands", from Oct. 26 to 29, 1987, K. Kishida et al. introduced a process with internal reforming, under the title of "Evaluation of Internal-Reforming Molten Carbonate Fuel cell for On-Site Application". Fuel cells with internal reforming are charged with chemically bound hydrogen, i.e., in the form of hydrocarbons. Thus, prepurified natural gas is utilized, for example, from which the fuel cells themselves produce the required hydrogen, utilizing the heat of conversion generated in the cell and the resultant steam. Excess hydrogen is also present in the anode waste gas in fuel cells of this type, so that the anode waste gas is frequently combusted to generate process steam.
The above mentioned uses of the anode waste gas, in one case for purposes of undergrate firing for the synthesis gas generator and, in another case, for the generation of process steam, generally do not efficiently utilize the hydrogen contained therein. Taking hydrogen for electrochemical oxidation in fuel cells will result in a conversion of 60 to 70% of its calorific value to electrical energy. In the generation of steam by the combustion of hydrogen only about 35% of the calorific value can be used.